IAEA-CN-104/59 LONG -TERM VADOSE ZONE PROCESSES AT THE NEVADA TEST SITE, U.S.A.
combined with appropriate levels of modeling are used to quantify the dominant transport processes in the shallow and deep vadose zone. Such models can then be applied with more confidence to other sites where radionuclides have been released and pose a potential threat to humans and ecosystems.
IAEA-CN-104/61 ISOTOPE CONTRIBUTION TO GEOCHEMICAL INVESTIGATIONS IN AQUIFER STORAGE AND RECOVERY
J. VANDERZALMa,d, C. LE GAL LA SALLEa,d, J. HUTSONa,d, P. DILLONb,d, P.
PAVELICb,d and R. MARTINc,d
aFlinders University of South Australia, Adelaide, South Australia, Australia
bCSIRO Land and Water, Adelaide, South Australia, Australia
cDepartment of Land, Water and Biodiversity Conservation, South Australia, Australia
dCentre for Groundwater Studies, South Australia, Australia
Aquifer Storage and Recovery (ASR) is an important resource management tool. An available water source; such as surface water, seasonal rainfall, or sewage effluent, is stored in a suitable aquifer for reuse when required to balance the seasonal demand for irrigation supply, thus relieving the pressure on groundwater resources. In ASR schemes, geochemical and biogeochemical reactions play an important role, impacting on both the aquifer matrix integrity and the recovered water quality. Understanding the driving processes that trigger these reactions is essential for determing the feasibility of new schemes and to adequately manage operating schemes. However the resulting geochemical signature is often due to a complex suite of reactions that is difficult to unravel. Few studies analyse the potential of isotopic tracers to contribute to the understanding of the biogeochemical reactions induced by ASR [1-4]. This paper investigates the potential of the stable isotopes of the water molecule, carbon-13 and carbon-14 and sulfur-34 to contribute to our understanding of the geochemical processes involved in reclaimed water ASR. The field trial at Bolivar, South Australia is investigating the viability of reclaimed water i.e. nutrient rich water, as an injectant.
The stable isotopes of the water molecule, can be used as conservative tracers to calculate the extent of mixing [1,3,4] as the stable isotopic signature of the reclaimed water and the native groundwater of the carbonate aquifer are significantly different. The δ2H and δ18O of native groundwater are fairly constant at -26±1 and -4.4±0.1 ‰ vs. SMOW respectively, while the injectant signature is more enriched and more variable, with δ2H ranging from –10.6 to –3.6
‰ and δ18O from –1.74 to –0.21 ‰. The variation in the injectant signature is attributed to seasonal variation in the degree of evaporation occurring in storage lagoons [4]. The seasonal signature variation is maintained as injectant penetrates observation wells 4m and 50m from the point of injection. This can be utilised in mixing calculations to constrain the portion of injected end-member that is penetrating an observation well and reduces uncertainty with using an average representation of the variable injectant quality.
The isotopes of carbon, carbon-13 and carbon-14, can help to characterise the source of oxidised organic matter and dissolved inorganic carbon [1-2]. In this study, carbon-13 and carbon-14 of TDIC are used to gain insight into two of the important processes involved with ASR, organic matter oxidation and calcite dissolution. The native groundwater δ13C signature is –11 ± 3‰ vs. PDB and the 14C activity ranges from 3-10 pMC. The injectant δ13C signature is more enriched, ranging from –7.0 to +0.1 ‰, and has a modern 14C activity of 100 ± 9 pmC. Upon injection, both organic matter oxidation and calcite dissolution are evident within
4m from the ASR well. This is reflected by a lowering of the carbon-14 activity in the 4m groundwater consistent with calcite dissolution (FIG. 1). This suggests the carbon-14 signature is sensitive to small additions of TDIC through reaction processes. The carbon isotopes behave differently upon breakthrough of injectant to the 50m radius, where the ambient signature dominates until the groundwater is 100% injectant. The final signature at 50m after full breakthrough, δ13C –8.1 ± 0.2 ‰ and 14C activity 58 ± 1 pmC, is somewhat lower than the injectant signature and may be attributed to additional reaction processes.
Sulfate isotopes, sulfur-34 and oxygen-18, can provide insight on sulfate reduction and pyrite oxidation reactions [1]. Sulfate reduction up to 1.5 mmol L-1 is evident in groundwater sampled from the ASR well during a period of aquifer storage, while sulfate concentrations 4m from the ASR well remain unchanged. Enrichment in residual sulfate, of around 12 ‰ vs SMOW, accompanied the decline in sulfate concentration, is typical of biologically mediated sulfate reduction. Stable sulfate and sulfur-34 signatures at the 4m observation well, indicate the sulfate reducing zone does not extend far from the ASR well.
0 20 40 60 80 100 120
-14 -12 -10 -8 -6 -4 -2 0 2
carbon-13 (‰ vs PDB)
carbon-14 (pMC)
injectant 4m observation well 50m observation well
100% injectant @ 50m
100% injectant @ 4m
native groundwater & <100% injectant @ 50m
FIG. 1. Carbon-14 versus carbon-13 of from observation wells 4m and 50m from the ASR well.
REFERENCES:
[1] HERZCEG, A.L, RATTRAY, K.J., DILLON, P.J., PAVELIC, P. & BARRY, K.E.
2000. Geochemical and isotopic tracers of recharge and reclamation of storm-water in an urban aquifer: Adelaide, South Australia. IAEA Project Res. Agreemt. AUL-10063.
[2] LE GAL LA SALLE, C., VANDERZALM, J., HUTSON, J.L., DILLON, P.J., PAVELIC, P., MARTIN, R. 2001. Investigation of the carbonate system in Aquifer Storage and Recovery: an isotopic approach. The 10th Water Rock Interaction Symposium, Vilasimius, Italia.
[3] STUYFZAND, P.J. 1998. Quality changes upon injection into anoxic aquifers in the Netherlands: Evaluation of 11 experiments. In “Third International Symposium on Artificial Recharge of Groundwater - TISAR 98” (J.H. Peters et al., ed.). A. A.
Balkema, Amsterdam, Netherlands.
[4] LE GAL LA SALLE, C., VANDERZALM, J. , HUTSON, J.L., DILLON, P.J., PAVELIC, P., MARTIN, R. 2002. Isotope contribution to geochemical investigations for aquifer storage and recovery. In “Management of Aquifer Recharge for Sustainability” - Proceedings of the 4th International Symposium on Artificial Recharge of Groundwater (P.J. Dillon, ed.), A. A. Balkema, Amsterdam, Netherlands.
IAEA-CN-104/66 TEMPORAL TRENDS IN THE CHEMICAL AND ISOTOPIC COMPOSITION OF SURFACE WATER NITRATE WITHIN THE OLDMAN RIVER BASIN, SOUTHERN ALBERTA, CANADA
L. ROCKa, B. MAYERa,b
aDepartment of Geology & Geophysics, University of Calgary, Canada
bDepartment of Physics & Astronomy, University of Calgary, Canada
Temporal variations of concentrations and isotope ratios of riverine nitrate from the Oldman River watershed were monitored over more than two years. This basin, located in Southern Alberta, Canada, has almost pristine headwaters in its western part and increased urban/industrial/agricultural activities in its eastern part. The objective of the study was to assess nitrate sources and their impact on surface water quality throughout the different seasons. Monthly sampling of the main stream of the Oldman River (OMR) and some of its tributaries (T) commenced in December of 2000, and a total of 14 sites (5 OMR & 9 T) were sampled until March 2002. Presently samples are taken from 21 sites every two months.
In the tributaries, [NO3- - N] ranged from < 0.003 to 8.810 mg/L, δ15Nnitrate values varied between -2.5 and +23.4‰ (Figs. 1a and 1b), and δ18Onitrate values ranged from -15.2 and +3.4‰. Tributaries located in the upstream-western portion of the watershed (< 100km) had low and invariable nitrate-N concentrations (≤ 0.5 mg/L) throughout the seasons. In contrast, some tributaries in the downstream-eastern part (> 100km) had high nitrate-N concentrations (> 1 mg/L) in the fall-winter and low concentrations in the spring-summer (Fig. 1a). Western sites (< 100km) had in general lower δ15Nnitrate values (~ +2‰) than eastern sites (~ +15‰), and the δ15Nnitrate values showed no seasonal variations at the respective sampling sites (Fig.
1b).
In the Oldman River, nitrate-N concentrations ranged from < 0.003 to 0.339 mg/L, δ15Nnitrate
values varied between -1 and +14‰ (Figs. 1c and 1d), and δ18Onitrate values varied between -10 and +6‰. Oldman River sites located in the upstream portion (< 100km) of the basin had low and constant [NO3- - N] (≤ 0.15 mg/L) throughout the seasons. Eastern sites had somewhat elevated nitrate-N concentrations in the fall-winter and low concentrations in the spring-summer (Fig. 1c). δ15Nnitrate values appeared to be constant with time at the respective sampling sites, but there was a trend of increasing δ15Nnitrate values from the western-upstream sites (~ +3‰) to the eastern-downstream sites (~ +8‰) (Fig. 1d).
Chemical and isotopic data suggest that nitrate in the western part of this watershed was mainly derived from soil nitrification (δ15Nnitrate < +5‰), whereas significant portions of nitrate in the urban/industrial/agricultural eastern part were derived from manure or sewage (δ15Nnitrate > +10‰) [1]. The latter anthropogenic sources caused high nitrate concentrations in the tributaries particularly in the non-irrigation season. This suggests that the local hydrology (e.g. water level in irrigation canals) has a major influence on the amount of agricultural nitrate reaching the streams and hence on their water quality. To what extent biological
Dec 00 Jan 01 Feb 01 Mar 01 Apr 01 May 01 Jun 01 Jul 01 Aug 01 Sep 01 Oct 01 Nov 01 Dec 01 Jan 02 Feb 02 Mar 02 -6
-2 2 6 10 14 18 22 26
δ15Nnitrate [%o]
b)
Winter 2002 Fall 2001 Summer 2001 Spring 2001 Winter 2001
Dec 00 Jan 01 Feb 01 Mar 01 Apr 01 May 01 Jun 01 Jul 01 Aug 01 Sep 01 Oct 01 Nov 01 Dec 01 Jan 02 Feb 02 Mar 02
1 3 5 7 9
[NO3- - N] mg/L
Winter 2002 Fall 2001 Summer 2001 Spring 2001 Winter 2001
a) 19 km 24 km 62 km 198 km 230 km 246 km 249 km 261 km 315 km
Dec 00 Jan 01 Feb 01 Mar 01 Apr 01 May 01 Jun 01 Jul 01 Aug 01 Sep 01 Oct 01 Nov 01 Dec 01 Jan 02 Feb 02 Mar 02
0.0 0.1 0.2 0.3 0.4
[NO3- - N] mg/L
c) 0 km 39 km 204 km 215 km 314 km
Winter 2002 Fall 2001 Summer 2001 Spring 2001 Winter 2001
Dec 00 Jan 01 Feb 01 Mar 01 Apr 01 May 01 Jun 01 Jul 01 Aug 01 Sep 01 Oct 01 Nov 01 Dec 01 Jan 02 Feb 02 Mar 02
-2 0 2 4 6 8 10 12 14
δ15Nnitrate [%o]
d)
Winter 2002 Fall 2001 Summer 2001 Spring 2001 Winter 2001
FIG. 2. a) [NO3- - N] versus sampling month for the tributary sites; b) δ15Nnitrate values versus sampling month for the tributary sites; c) [NO3- - N] versus sampling month for the Oldman River sites; d) δ15Nnitrate values versus sampling month for the Oldman River sites; hatched area represents
irrigation season; distances in km downstream of Oldman River site (0 km).
REFERENCE:
[1] ROCK, L., and MAYER, B. 2002, Isotopic Assessment of Sources of Surface Water Nitrate within the Oldman River Basin, Southern Alberta, Canada, BIOGEOMON, 4th International Symposium on Ecosystem Behaviour, Aug. 17-21, The University of Reading, UK, Book of Conference Abstracts, p. 203.
IAEA-CN-104/68 RESULTS OF LONG TERM INVESTIGATIONS ON 18O IN THE UNSATURATED ZONE IN COMPARISON TO THE RESULTS OF TRACING EXPERIMENTS AND NUMERICAL MODELING